Reloading .223/556

Perhaps two of the most popular cartridges in the reloading brass
world is the .223 Remington and 5.56 NATO. These have become so popular in
today’s shooting world mainly due to the popularity of AR15 sales. If one
purchases an AR15 chambered in one of these two calibers, you can shoot either
cartridge with no apparent problems.

Case cartridges for the 5.56 NATO (5.56 x 45 mm) and the .223
Remington are very similar.

For all practical purposes, the external dimensions are
identical, and one can effectively say that there exists no difference between
the two. Both are designed to accommodate a .224 inch (5.56 mm) bullet between
40 to 85 grains, and both offer fairly equal performance in range and accuracy.

From an internal standpoint, however, there are some differences
to be aware of – most notably, the case capacity. Simply
put, the 5.56 case has thicker brass walls to handle higher pressures and,
therefore, has less interior volume than the .223 case. This is especially
important to reloaders because the powder loads are affected by these different
case capacities. Its important to always check your reloading manuals when
preparing these cases to ensure your load data is for the specific reloading
brass cartridge you are reloading.

Beyond the interior case capacity, the significant
difference between the 5.56 NATO and the .223 Remington case cartridges is
found in the barrel chambers in which they are used. Although both rounds will
fit into barrel chambers designed for either cartridge, the lead for each is
very different.

The lead of a barrel is the area in front of the chamber,
but before where the rifling begins. For the .223 Remington, the lead
specification (SAAMI) is 0.085 inches. For the 5.56, the lead specification
(SAAMI) is 0.162 inches, or, almost double. This difference is notable.

Because standard 5.56 NATO rounds are produced with higher
pressure loads, if it’s fired in a rifle designed for a .223 Remington, the
shorter lead in the .223 Remington chamber will further increase the pressure, potentially
to an unsafe level. Additionally, manufactured 5.56 rounds are made for the
longer lead and, therefore, may have the bullet seated differently. If the
bullet from a 5.56 makes contact with the rifling in a chamber designed for a
.223 Remington prior to being fired, this too can cause unwanted pressure
increases.

Conversely, firing a .223 Remington round through a 5.56
chamber is fine, however, the ballistics of the bullet may be affected through
a lower velocity. Also, depending on the shooting situation you are reloading
for, you might find that you use a different powder, or a powder with a
different burning rate, to make up for the difference in case capacity.

This could also require you to use a different size bullet
to make up the change in velocity. In summary, the 5.56 NATO case cartridge and
the .223 Remington case cartridge are interchangeable.

Nevertheless, reloaders need to be aware of the powder loads
and bullet seating that will correctly accommodate the chamber of the rounds
they’re being prepared for. It is important to remember, though, that loading
data is NOT interchangeable. Never use reloading data for the .223 when
reloading 5.56 brass, or vice versa.